Common optical discs can be classified into two categories according to the number of layers: single layer or dual layer. Single layer optical discs include CD, VCD, DVD-5 and DVD-10 discs. Dual layer optical discs include DVD-9, DVD-18 and SACD (super audio compact disk) discs. When an optical drive wants to read dual layer optical discs, the pick-up head of the optical drive should be able to move the laser light spot between two different layers. Therefore, a layer jump process is required to let laser light spot move from an initial layer to another target layer.
The two recording layers of a dual layer optical disc are usually called layer 0 and layer 1. The object of a layer jump process is to let the laser light spot leave from layer 0 to layer 1 or vice versa. Generally speaking, the control force involving a layer jump process can be divided into three parts: a control force moving the pick-up head from a natural equilibrium position to an initial layer; a kick force letting the pick-up head leave from the initial layer; and a brake force keeping the pick-up head in the target layer.
This conventional layer jump control method has its problems. The distance between the two recording layers of dual layer optical discs like DVD-9, DVD-18 and SACD is 55±15 um. The linear control region of each layer is ±1 um. Obviously, the linear control region is very small, but the variation of layer distance between different recording layers is comparatively large. Therefore, if the same control force, kick force and brake force are used to perform the same layer jump process when the natural equilibrium position of a lens is used as the jump position, the problem of unsuccessful layer jump may occur for some optical discs.
Either U.S Pub. App. No. 2003/0076755 or R.O.C. Pub. App. No. 509930 has proposed a solution to this problem. In this method, as shown in FIG. 1, the whole layer jump process is divided into four processes: a kicking process, a holding process, a braking process and a waiting process. This method is mainly characterized in that a focusing control signal is processed by a low-pass filter to produce a layer distance balancing signal. When a layer jump control signal is received to start a layer jump process, the produced layer distance balancing signal plus a kicking signal and a braking signal is used as the basis. Each time, the layer jump process lets the lens jump from a new natural equilibrium position to a target layer. The whole layer jump process will be simpler and easily controlled.
The main drawback of this method is that it can't handle the wobble effect caused by high-speed rotation of optical discs. Optical discs will wobble up and down relative to the natural balanced point of the lens due to bad positioning or bad manufacturing of optical disc.
For optical discs at low rotation speed, the wobble effect is a low-frequency response. For instance, the wobble period of a 2× DVD drive is about 21.7 ms. The layer jump process is comparatively fast, having a period of about 1.4 ms. Therefore, the wobble effect won't affect natural balance of lens for layer jump process at low rotation speed. Therefore, the produced layer distance balancing signal after the low-pass filter is still useful for wobble at low rotation speed.
FIG. 2 shows variation of the natural equilibrium position of lens for wobble at high rotation speed. For a 12× DVD drive, the wobble period is about 9.3 ms. Vibration of the DVD drive itself will also intensify the wobble effect of optical disc. As shown in FIG. 2, the control force of lens will vary along with the wobble period. The shorter the wobble period, the influence of wobble effect upon the layer jump process can't be neglected more. The control force of natural equilibrium position of lens during the layer jump process is not equal to that before layer jump, resulting in the existence of an inertial acceleration during the layer jump process. The magnitude of this acceleration depends on the jump point of lens from start. The control method of the above disclosures will result in easy failure and instability of layer jump of DVD disc.